Display devices are employed in diverse applications. For example, televisions, computers, telephones and personal digital assistants (PDAs) all have a display. In some display applications, it is useful to provide a large viewing area, while in others it is useful to provide a small viewing area. In many applications it is desirable for the display to be relatively thin. Similar demands are often placed on lighting structures. For example, many light sources in liquid crystal displays (LCDs) are usefully thin thereby fostering a thin LCD device.
There exists a variety of light source technologies available for display and lighting applications. One light source technology that has received attention in the display and lighting industries is the organic light emitting device/diode (OLED). OLEDs are often made from electroluminescent polymers and small-molecule structures.
The basic OLED structure includes an organic light emitting material disposed between an anode and a cathode, which are normally disposed on opposing sides of the material. When current is supplied to the organic light emitting material, light is given off through the radiative decay of excitons formed by the recombination of electrons from the cathode and holes from the anode. As can be readily appreciated, the light emitted from the OLED can be used in lighting (e.g., backlight) and display device applications.
In order to form a display device, a plurality of anodes and cathodes are driven by a thin film transistor (TFT) circuit. The anodes and cathodes are formed in an array, which in turn provides an array of picture elements (pixels) from the OLED material. Display images can then be formed by the selective application of current through the anodes and cathodes, which are driven by the TFT circuit.
Lighting applications also may be based on the driving of the OLEDs via cathodes and anodes. Of course, the selective application of current and array structures comprised of cathodes and anodes needed to realize pixels in displays is foregone. Rather, a less complex anode and cathode structure may be used.
While OLED-based displays and lighting sources are viable alternatives to known display and lighting technologies, there are shortcomings to known OLED-based structures. For example, known OLED structures are not conducive to efficient light output (luminance). In fact, in many known OLED structures, as much as 80% of the light energy emitted by the OLEDs is trapped in the device. In order to improve the amount of light emitted from the device, it is necessary to drive the OLEDs at relatively high current levels. However, these relatively high drive currents have a deleterious impact on lifetime of the OLED. To wit, a unit increase in the drive current results in a unit exponential decrease in device lifetime.
There are many factors that can adversely impact the amount of light transmitted from the OLED to the external environment (e.g., viewing surface of a display). One such factor is total internal reflection (TIR). As is well known, many organic materials used in OLEDs have a relatively high index of refraction (norg). For example, norg>1.8. As such, the organic material (and other layers surrounding the organic layer) forms a waveguide, which can trap a significant portion of the light generated by the OLED. Clearly, this light will not be transmitted to the external environment. Illustratively, if the organic layer interfaces air, the classical limit of the emitted light is given by: 1/(2 norg)2. As such only approximately 17% of the emitted light emitted by the OLED material reaches the air.
While certain attempts to reduce internal reflection have been made, these have met with mixed results. As such, there remains a need to improve the light output efficiency of OLEDs and thus increase the lifetime of such devices.